Process for removing dirt from wood pulp



Dec. 21, 1954 K. A. CRAIG ETAL 2,697,384

PROCESS FOR REMOVING DIRT FROM WOOD PULP Filed Jan. 14. 1950 United.states Patent PRUCESS FOR REMQVING DIRT FROM W091) PULP Kenneth A.Craig, Appleton, and Charl Louis Roux do Wet, Neenah, Wis, assignorstoPaper Patents Company, Neenah, Wis, a corporation of WisconsinApplication January 14, 1950, Serial No. 138,556

4 Claims. (Cl. 92-2(l) The present invention relates to an improvedprocess for removing various types of dirt specks, including grit, pipescale, bark and knot specks, etc. from wood pulp. More specifically, thepresent invention is directed to the removal from mechanical orgroundwood pulp of that type of dirt which is formed in situ in thegrindingprocess, i. e. dark colored specks which are formed by thecomniinution of knots and bark present on and imbedded in the pulp woodlogs. I

Mechanical or groundwood pulp, unlike chemical pulps such as the sulfiteand kraft cooked pulps, contains in addition to the cellulose fibers allof the wood, including the complex lignin and hemi-cellulose materials.It is produced by pressing a peeled or barked pulp wood log against thesurface of a revolving stone, the rotating axis of the stone beingsubstantially parallel with the grain or fiber direction of the log. Theabrasive or cutting action of the grit of the stone combined with thepressure and temperatures created on grinding results in the cutting andbreaking of the wood from the parent log. In the process the entire log,including the knots, etc., is reduced to fine fibrous fragments. Theknots, at least at their outer periphery, are most generally discoloredwith stains or pitchy material which when reduced to a fine particlesize results in what is commonly called dirt specks in the mechanicalpulp. With certain species used for groundwood, notably poplar, theknots are frequently partially encased with bark which results in thegroundwood pulp being further contaminated with additional dark coloreddirt specks.

in contrast to groundwood, the chemical pulps do not generally containexcessive amounts of dirt, hence do not require special treatments toremove the dirt specks com ing from the above described knots and bark.In producing a chemical pulp, e. g. a sulfite pulp, the logs are chippedprior to the chemical cooking process with the result that many of theknots are broken out as whole knots or at least as large pieces. Theselarge pieces are removed by the chip screens and do not get into thedigester where the chips are cooked. 1n the event that small chips orpieces of knots and bark do get into the digester, they go through thecooking process more or less unchanged as far as size is concerned andare removed from the pulp by the knotter screens. The dirt particles inchemically cooked pulp that do pass through the screens and finallyremain in the pulp are relatively few in number as compared with theamount of dirt which would have resulted from producing groundwood pulpfrom the same quality of pulp wood logs. Also, chemically cooked dirtspecks, unlike uncooked groundwood dirt specks, are renderedunnoticeable during the bleaching operations used to bleach chemicalpulps. I

In order to produce a groundwood pulp which is sufficiently clean, i. e.free of dirt specks, to be used in the better grade of papers ortissues, it is necessary (a) to remove the contaminating source prior tothe groundwood process or (b) to remove the dirt from the pulp after thewood with its associated dirt-forming barky knots has gone through thegrinding process. The removal of the sources of the dirt such as by theselection of only the highest quality of wood for use in the groundwoodprocess is very costly and wasteful of wood and has proven commerciallyimpractical. In view of this, attempts have been made to remove knotsand their encasing bark by various cleaning operations in the wood roomprior to grinding. Knot saws and knot drills have been tried, for

in the groundwood process, also contains an extremely large number ofpin knots, the removal of even a majority of which would be impractical.Poplar, another pulp wood species, contains fewer knots than spruce butthe knots are generally much larger and frequently are ingrown so thatonly a bump appears on the surface, masking a large diameter knot withencrusted bark. In addition to requiring much manipulation resulting inexcessive labor costs, wood cleaning operations, like selection, havealso proven to be wasteful of wood. Practical operating experience hasdemonstrated that it is not possible to produce a groundwood of thedesired dirt-free quality level by either selection or cleaning of thewood prior to grinding in the groundwood process. s-

in view of the above, the art has made many attempts to remove or bleachout the dirt specks from groundwood per se, i. e. from the wood afterthe grinding operation. These have included screening operations and inparticular chemical treatments. The use of screens has provenunsatisfactory as the knots and bark are reduced to a fine state ofsub-division during grinding and the resulting dirt specks have beenfound to pass through the screens with the finely divided clean wood.The chemical treatments employed have been of the bleaching type.Illustrative procedures are described in the following U. S. patents:1,991,824 and 2,450,034 (zinc hydrosulfite); 2,187,016 and 2,199,376(alkaline peroxide); and 2,474,034 (alkaline hypochlorite). Theprocesses described in these patents, as well as the multiple bleachingstage operations used to bleach chemical pulp, bleach the fibers andimprove the brightness or color of the groundwood but do not remove oreven appreciably improve the color of the knot and bark specks. Thealkaline hypochlorite processdescribed in Patent 2,474,034, for example,removes (bleaches) dirt specks caused by heart or fungi rot but does notremove or bleach out the color of the gelitively large objectionablespecks caused by knots and The quality level of groundwood from thestandpoint of the number of permissible dirt particles per unit areadepends upon the size of the dirt specks and on the end use of thefinished sheet of paper containing the ground wood. It the groundwood isto be used in the production of a high grade white printing paper, thefewer dirt specks it contains the better. In specifying dirt sizes 21'Dirt Estimation Chart has been developed by da Clarke and is now usedgenerally in the paper industry. In this classification chart variousshapes are grouped together according to the area of the dirt particle.There are four classifications having the range of areas given below:

Class 10.040.1 min. Class 20.1O.5 mm? Class 3-O.5-1.5 mm? Class 4'1.55.0mm.

in the better classes of book paper which are used in magazinepublications carrying multi-colored advertisements, an appreciablenumber of class 1 specks is permissible, a few class 2 specks areallowable in an 8 /2xll sheet, while classes 3 and 4 dirt specks shouldbe entirely absent. Experience in producing groundwood, especially frompoplar which contains bark encased knots, has established that the pulpcontains some class 3 and 4 dirt specks and a very large number of class1 and 2 specks.

During the research investigation leading to the pres ent invention,attempts using various physical and chemical methods were tried as inthe past to remove or bleach out the dirt specks from groundwood. Thephysical methods which were tried were based upon gravitational andcentrifugal principles. Processes employing these principles provedsatisfactory for removing pieces of grit, pipe scale and the like whichhave densities appreciably different than the groundwood pulp. Theseprocesses, however, failed to solve the primary problem as they did notgive the desired separation of particles of bark and knots from thegroundwood. Careful measurements of the densities of encased bark andknots and TABLE I Specific gravity Encased bark 1.51 Knots 1.50 100%sound wood 1.50

An analysis of this data makes a separation based upon densitydifferences appear impossible. Experience has borne this out as none ofthe physical processes known heretofore give the desired degree ofcleanliness, i. e. remove the objectionable bark and knot specks fromgroundwood pulp.

The attempts to solve the problem by chemical means have likewise beenunsuccessful. Illustrative results obtained using 100 per cent poplarbark and knot particles with the alkaline hypochlorite bleaching processdescribed in Patent No. 2,474,034 using 30 per cent available chlorine,are tabulated below as brightness values as measured on the GeneralElectric Brightness Tester.

TABLE II The 30 per cent available chlorine employed is about 2.5-3.0times the quantity ordinarily recommended for bleaching poplar. Withless available chlorine the improvements in brightness values, if any,were less than those obtained using the abnormally large quantities ofavailable chlorine. It is obvious from these results, as groundwoodshould have a brightness of 70-78 to be used in the better grades ofprinting papers, that bleached bark and knots with brightness values ofabout 11 and 28, respectively, will show up as dirt specks or blemishesand present a very unsightly'appearance in the finished sheet.

The attempts to solve this problem were made by the paper industrybecause groundwood imparts desirable printing properties to paper and isa preferred type of pulp for many uses. It is also relatively easy toprepare and bleach and compared to chemical pulp, is relatively low incost. However, up until the present invention, groundwood pulps, andparticularly groundwood pulps prepared from low cost woods such aspoplar which provides a relatively strong pulp of superior opacity, havenot been given widespread use in the high grade papers, better printingpapers, etc., due to the failure of the art to provide a commerciallypractical method for removing objectionable bark and knot specks frompulp.

The principal object of the present invention is to provide acommercially practical process for separating dirt specks from woodpulp.

Another object of the present invention is to provide a process formaking groundwood which is contaminated with dirt specks and notsuitable for use in high grade papers, adaptable for use in themanufacture of such papers.

A more specific object of the present invention is to provide animproved process for separating discolored bark and knot specks, formedin situ during the grinding operation, from groundwood.

Other objects of the present invention will be apparent as thedescription proceeds.

The process of the present invention is based on the discovery that in adilute aqueous wood pulp, and particularly a groundwood pulp suspension,finely divided air bubbles are selectively occluded by or attached tothe pulp and not to dirt specks, including dirt specks derived from barkand knots formed in situ during the grinding operation. The occlusion ofthe bubbles on the pulp has been found to be adequate to impart atemporary Ypressure. number of extremely fine bubbles of air. Theaerated buoyancy to the pulp causing it to float to the surface of thesuspension.

The reason for the selective occlusion of air bubbles on the pulpresulting in increased buoyancy is not fully understood. The phenomenonis believed, however, to be at least in part a physical process as it isnot permanent. A chemical process may also be involved, particularlywith groundwood pulp which is made up of lignin and other complexsubstances in addition to cellulose fibers.

The air bubbles used in the process of the present invention may beformed in or introduced into the aqueous pulp suspension by variousmethods known in the art. In one of the preferred methods, the airbubbles are formed by dissolving air under pressure in the pulpsuspension by maintaining the pulp under pressure in the presence ofexcess air followed by a reduction of the pressure with or without theapplication of vacuum. This results in the formation or release ofextremely small bubbles of air in situ and may be readily carried out ina pressurized tank of the type discussed in detail below. The air mayalso be introduced into the suspension by rapidly agitating or beatingof the pulp suspension with stirring or mixing equipment which producesa vortex and brings air into the suspension. Other means of airdispersal in place of the pressure or agitation methods may also beused, as, for example, the introduction of the air into the suspensionthrough porous plates and the like, cascading of the pulp suspension,etc. Any type of aerating equipment may be employed as long as it bringsfine air bubbles into intimate contact with the pulp in the pulpsuspension.

The density differential between the pulp with occluded air and the dirtparticles has been found to be sufficiently wide and to persist for asufficiently long time to permit their separation with equipment basedupon gravitational or centrifugal principles or a combination of bothprinciples. As previously stated, these principles cannot be utilizedsatisfactorily for separating pulp from bark and knot dirt without theuse of air since under these conditions the densities of the pulp anddirt are too close. (See Table I above.)

The gravitational principles which may be employed to effect theseparation after the treatment of the pulp suspension with. air aresettling basins, rifilers and various types of save-alls. A well knownsave-all is the Sveen-Pedersen Saveall employing a pressurized tank andflotation chamber described in detail at pages 37-41 in the Paper TradeJournal of August 11, 1949. The centrifugal types of equipment which maybe used to effect separation of the treated pulp from dirt include lowspeed centrifuges, and the Vortrap and Dirtec type of equipmentdescribed in TAPPI, vol. 32, No. 10, pages 454457 of October 1949, andreferences cited therein.

The single figure of the drawing is a schematic view illustrating onemethod of practicing the invention.

The following examples will serve to illustrate the present invention.

EXAMPLE I This example employs the equipment illustrated in the drawing.An aqueous slurry of unbleached poplar groundwood at about 0.25%consistency and contaminated with bark and knot dirt specks enters thesystem through conduit or pipe 10 and, except for the portion recycled,is pumped continuously by pump 11 through conduits 12 and 12 into a tengallon retention or pressure tank 13 where it is maintained at apressure of about lbs./in. sq. for about 1.5 minutes. The portion of theslurry recycled and which may be controlled by valve 14 passes by way ofpipe 15 to pump 11. Air is injected into the stock at the rate of about0.15 ft. /min. by means of a venturi 16 in line 15. As the stock entersthe retention tank 13 at 17 and leaves at the other end through pipes 18and 18', there is little or no channeling. The rate of fiow is alsoabout 6.5 gals/min. which is adequate to keep the stock under pressurein the tank for about 1.5 minutes.

The stock flows from the pressure tank 13 through pipes 18 and 18 andpipe 19 and as valve 27 is in opened position and valve 28 is in closedposition, then flows by way of pipe 19' through an inlet disc valve ororifice shown at 20 into a surrounding compartment or inlet chamber 21where the pressure is reduced to atmospheric This results in the releasein situ of a large grade papers.

EXAMPLE II About three parts by weight of unbleached poplar groundwoodcontaminated with bark and knot dirt specks is first suspended in amixing tank in about 1,000 parts of water. Air is next whipped into theresulting slurry which is at about a 0.3 per cent consistency bystirring for about one minute with a high speed mixer. The aerated stockis then pumped from the mixing tank to a settling tank where the dirtspecks settle to the bottom and the pulp with occluded air bubbles risesto the top and is removed. While the use of this equipment is notgenerally preferred in commercial operations, tests have demonstratedthat it is adequate to remove the objectionable class 4 specks, about6070 per cent of the class 3 specks and 45-50 per cent of the class 2specks.

EXAMPLE III This example employs the pressurized tank and flotationchamber of Example I and follows the same procedure as Example I. Inplace of unbleached groundwood used in Example I, the present exampleemploys bark and knot contaminated alkaline hypochlorite bleached poplargroundwood bleached in accordance with the process described in U. S.Patent 2,474,034. The bleached groundwood at a consistency of about 0.2%is first aerated in the pressurized tank right after the completion ofthe alkaline bleach, i. e. prior to acidification or neutralization,while at an alkaline pH of about 9l0. The clean groundwood recoveredfrom the flotation chamber is substantially free from objectionable dirtspecks and is of the type desired for use in the manufacture of thebetter grades of printing papers.

EXAMPLE IV This example employs the pressurized tank of Example I incombination with a Vortrap. A dilute suspension (0.4% consistency) ofunbleached poplar groundwood contaminated with bark and knot dirt specksis first aerated in the pressurized tank 13 in accordance with theprocedure of Example I. As valve 27 is placed in closed position andvalve 28 is placed in open position, the stock instead of going to theinlet and flotation chambers as in Example I, passes through pipes 18,18' and 19 and then pipe 29 where it is fed directly into a 1" Vortrap(not shown) which passes 25-30 gals. per minute at an inlet pressure of60 lbs/sq. in. The clean pulp collected at the discharge of the Vortrapis of the same quality as that obtained in Example I.

Unbleached groundwood which has been properly aerated will float equallywell in a flotation chamber under neutral, acidic or mildly alkalineconditions. However, as unbleached groundwood tends to become brownishin color in the presence of alkali, pH values of above about 8 should beavoided when treating unbleached groundwood. Alkaline bleachedgroundwood is less sensitive to color change than unbleached groundwoodand as it floats more rapidly at the alkaline pH values, the use of a pHrange of about 811 is preferred for the flotation of alkaline bleachedgroundwood. In practice it has been found best to aerate the bleachedgroundwood after the completion of the alkaline bleach and before thesecond stage or acidifying or neutralizing step of the bleachingprocess. Acids such as sulfur dioxide, hydrochloric acid, etc. andalkalies such as caustic soda may be added to the pulp to obtain thedesired pH.

Investigations have demonstrated with gravitational separation as in thecase of a flotation chamber, that the consistency of the pulp shouldpreferably be about 0.20-0.25 per cent and should not exceed about 0.35per cent for optimum results. With centrifugal equipment such as theVortrap, higher consistencies up to about one per cent may be employed.Lower consistencies may also be employed with either gravitational orcentrifugal separation, although the use of highly diluted pension insaid release zone effects suspensions with consistencies much-below 0.20has been found impractical in mill operations.

Investigations have also demonstrated that the aeration of'the pulp andsubsequent separation of dirt from pulp may be advantageously carriedout at the normal temperatures encountered with slush groundwood in thepaper 3050 C. Lower temperatures may be used, as for example when lappedpulp is repulped with fresh or cold water, but under these conditionsthe desired flotation of.

the aerated pulp takes place more slowly.

The time required for obtaining the optimum amount of aeration of thegroundwood slurry depends upon the aeration method employed, temperatureof the pulp, etc. In a pressurized retention tank of the type employedin Example I at about 40-80 lbs./in. sq., and a temperature of about3050 C., the stock should remain in the tank for at least about oneminute. Optimum aeration times for particular equipment may be readilyascertained by preliminary experimental test. Also, although theinvention is applicable to other types of pulp, it is of special valuefor treatment of groundwood pulp as the scribed herein.

The process of the present invention may be carried out without addedchemicals. Small amounts of .sur-. face active agents which lower theinterfacial tension, such as l5 .per cent (based on the weight of theoven dry pulp) of the aliphatic alcohol sulfate wetting or dispersingagents sold under the trade name DuponoF ma, however, be added to thepulp before aeration if desired to effect a more rapid flotation of thepulp. Small amounts of flocculating agents such as 1-5 per cent (basedon the weight of the oven dry pulp) of alum either alone or inconjunction with the surface active agents may also be used if desiredto increase the rate of flotation of the pulp. The use of addedchemicals, however, is not necessary in the present invention, and inany event, the use of chemicals which produce froth or foam should beavoided as they carry the bark and knot specks along with the pulp andprevent the desired separation. The use of chemicals which produce frothor foam should also be avoided as they give trouble in subsequentpapermaking operations.

We claim:

1. The method of improving the characteristics of groundwood pulpcontaining wood fiber contaminated with dirt particles, includingdiscolored bark and knot specks, which comprises eflfecting the additionof air to a dilute aqueous suspension of said pulp, increasing thepressure on said suspension, and then passing said suspension into apressure release zone, the amount of air introduced into said suspensionprior to movement of said suspension into said pressure release zonebeing substantially in excess of the amount of air which the suspensionwill retain under the conditions prevailing in said release zone,whereby the release of air in said susflotation of said wood fiberswithin said suspension and movement of dirt particles in the oppositedirection, and separating the floated fibers from the contaminants.

2. The method of improving the characteristics of groundwood pulpcontaining wood fibers contaminated with dirt particles, includingdiscolored bark and knot specks, which comprises aerating a dilute,aqueous suspension of said pulp by entraining air in said suspension andby passing said suspension and entrained air into a pressure zone,passing the resulting air-containing suspension into a second zone whichis maintained under such conditions that the maximum pressure in saidsuspension while it is in said second zone is substantially less thanthe pressure existing in said pressure zone, thereby efiecting releaseof very small air bubbles through said pulp suspension and flotation ofsaid fibers within said suspension, and separating the floated fibersfrom the remaining contaminants.

3. The method of improving the characteristics of poplar groundwood pulpcontaining wood fibers contaminated with dirt particles, including barkand knot specks, which comprises eifecting the addition of air to adilute aqueous suspension of said pulp by entraining air in saidsuspension and by passing said suspension and entrained air into apressure zone which is maintained at a pressure within the range of fromabout 40 to 80 pounds per square inch, passing the resultingair-containing suspension into a second zone which is maintained undersuch mill. These temperatures range from aboutconditions that themaximum pressure in said suspension while it is in said second zone issubstantially less than the pressure existing in said pressure zone,thereby effecting the release of very small air bubbles through saidsuspension and flotation of said wood fiber within said suspension, theamount of air added to said suspension prior to movement of saidsuspension into said second zone being substantially in excess of theamount of air which the suspension will retain under the conditionsprevailing in said second zone, and separating the floated fiber fromthe remaining contaminants.

4. The method of improving the characteristics of groundwood pulpcontaining wood fibers contaminated with dirt particles, includingdiscolored bark and knot specks of such size relative to the size of thefiber that it is not practical to remove the specks by screening, whichcomprises effecting the addition of air to a dilute, aqueous suspensionof said pulp by entraining air in said suspension and by passing saidsuspension and entrained air into a pressure zone sure within the rangeof from about 40 to 80 pounds per square inch, said suspension beingmaintained in said pressure zone at least about one minute, passing theaircontaining suspension into a pressure release zone maintained undersuch conditions that the maximum pressure in said suspension, while itis in said pressure release zone, is substantially less than thepressure existing in which is maintained at a pressaid pressure zone,thereby effecting release of very small air bubbles throughout saidsuspension, flotation of wood fibers within said suspension, andmovement of'the contaminating dirt particles in a direction opposite tothe direction of movement of said wood fibers, the amount of air addedto said suspension prior to movement of said suspension into saidpressure release zone being substantially in excess of the amount of airwhich the suspension the conditions prevailing in said presandthereafter separating the floated will retain under References Cited inthe file of this patent UNITED STATES PATENTS Number Name Date 864,856Norris Sept. 3, 1907 1,226,333 Hoskins May 15, 1917 1,312,976 Groch Aug.12, 1919 1,988,416 Freeman Jan. 15, 1935 2,596,015 Dunwody May 6, 1952FOREIGN PATENTS Number Country Date 277,310 Great Britain June 14, 1928559,892 Great Britain Mar. 9, 1944

